In ultrasound imaging, the internal body is imaged by first transmitting an ultrasound wave towards an area of interest and then receiving the reflections generated as the wave passes through the internal body tissue at various depths. The ultrasound wave is typically generated and received using one or more ultrasound transducers. Imaging hardware and/or software within an imaging system stores the set of reflections, or echoes, received from each ultrasound transmission as an echogenic data sets, also referred to as an echo record or scan-line. This echogenic data set is used to generate a visual image displaying body features at various depths, the existence of which is correlated to time echoes are received and the echo's relative amplitude. Echoes received earlier in time are displayed as shallow features located close to the transducer, while echoes received later in time are shown as deeper features.
Certain portions in the body, such as bone, have a higher echogenicity than other, softer portions such as muscle or blood. These highly echogenic portions reflect more of the incident ultrasonic wave and create echoes having a greater amplitude than portions having a relatively low echogenicity. In the image, each echo is assigned a brightness value based on the level of the echo amplitude. This provides the viewer with additional information regarding the composition of the portions of the body located within the region of interest.
However, the ultrasound wave diminishes in amplitude, or attenuates, as it travels through the body tissue. As a result, the echoes generated by portions of the body located close to the transducer are relatively stronger than those generated at a greater distance from the transducer. If left uncorrected, the resulting image can incorrectly represent the objective echogenicity of the various body structures. An uncorrected image might even exhibit excessive brightness in the region close to the transducer, while leaving the rest of the image dark.
An example of an uncorrected ultrasound image 102 is depicted in FIG. 1A. This exemplary image 102 is representative of one obtained with an intravascular imaging device, such as a catheter and the like, placed within a blood vessel. Shown within the field 103 of image 102 is the catheter outer wall 104, a blood vessel wall 105 and various tissue features 106-108 in and around the vessel wall 105. Here, it can be seen that the vessel wall 105 is relatively brighter than the surrounding tissue features 106-108 due to the attenuation of the transmitted ultrasound signal.
To compensate for this, conventional ultrasound imaging systems employ special hardware and/or software in the signal path to multiply the amplitude of each incoming echo signal by a time-varying amplification factor that amplifies echoes to a greater degree the later in time that they are received. The operation of applying this time-varying amplification is often referred to as “Time Gain Compensation” or TGC. A manual TGC input interface (consisting of a number of sliding controls, one for each range of depths) is typically provided in ultrasound systems to allow the user to adjust the time-varying amplification to achieve a desired result. An example of a time-gain compensated ultrasound image 102′ is depicted in FIG. 1B. Here, it can be seen that the vessel wall 105 and the surrounding tissue features 106-108 all have comparable brightness levels as a result of the TGC.
Recently, an automatic TGC technique was proposed in U.S. Pat. No. 6,743,174 entitled “Ultrasonic diagnostic imaging system with automatically controlled contrast and brightness,” which is fully incorporated herein by reference. This technique, targeted for use with an external ultrasound device, allows a user to time-gain compensate an image without having to manually adjust the gain levels for each depth. However, this technique still requires user-initiated input to initialize the TGC settings and therefore is not fully automatic. Also, this technique relies on predetermined gain levels stored in memory to serve as baseline gain values. Only after these predetermined gain values are applied does the technique attempt to determine what additional correction is necessary. Furthermore, this technique can only determine one gain value for each depth in the image and is incapable of determining a gain value for each depth along the individual scan-lines within the image.
Accordingly, improved automatic TCG systems and methods are needed that can overcome the shortcomings of conventional techniques while at the same time providing greater performance.